I've just finished the first reading of the deluxe edition (i haven't play a single scenario and english is not my mother tongue).
I did not really understood everything about the rules of Automatic Fire/rapid Attack and multiple action.

First of all, page 66 on the Multiple Actions paragraph you can find "A character can't fire more than his weapon's rate of fire".
So with a multiple action you could with a rifle with RoF2 shout 2 single shots with -2 to your rolls.

With the paragraph "Ranged Attacks" you learn that you can shoot (without malus) as much shoots as your weapon's RoF. So if I have RoF3 I shoot 3 times per round (with 3 ammos).

p.70 "automatic fire" says (on the opposite) that to use the automatic fire abality of the gun (which means - i think - shooting more than one bullet) you would have a -2 to your rolls. So here with the same weapon if i want to shoot 3 times, i will have a malus of -2 (and lose 6 ammos). But the paragraph from ranged attacks said the opposite !

p. 75 "rapid attack" shows that only with a semi automatic or an revolver you can fire 6 shots with a penalty of -4. But it contradicts the rule from page 67 about ranged attacks.
Why is it not possible with a full automatic gun ?

So,
can shoot more than the RoF of a gun with a "rapid attack" ?
do we have a penalty if we shoot more than one time per round ?

First of all, page 66 on the Multiple Actions paragraph you can find "A character can't fire more than his weapon's rate of fire".
So with a multiple action you could with a rifle with RoF2 shout 2 single shots with -2 to your rolls.

Actually, it only states that a weapon can't attack more than it's Rate of Fire; it does not state that such attacks work as normal multiple actions. That's why Rate of Fire is specifically covered on the next page...

Amphi2 wrote:

With the paragraph "Ranged Attacks" you learn that you can shoot (without malus) as much shoots as your weapon's RoF. So if I have RoF3 I shoot 3 times per round (with 3 ammos).

Rate of Fire only discusses how the dice are rolled to make an attack with a weapon with a ROF greater than 1, clarifying that the attacker rolls one Shooting die for each point of ROF but only one Wild Die, and that all of the attacks are considering a single action.

Nothing in there discusses the penalty for such attacks or the amount of ammunition used. That again is covered under the Automatic Fire that comes next...

Amphi2 wrote:

p.70 "automatic fire" says (on the opposite) that to use the automatic fire abality of the gun (which means - i think - shooting more than one bullet) you would have a -2 to your rolls. So here with the same weapon if i want to shoot 3 times, i will have a malus of -2 (and lose 6 ammos). But the paragraph from ranged attacks said the opposite !

As noted, it didn't say the opposite; it simply didn't cover the penalty or ammunition usage in discussing how the action works for ROF.

Unless specifically stated differently in a weapon's description, the -2 penalty and ammunition usage applies to any weapon with a ROF of 2+ when used for multiple attacks.

Amphi2 wrote:

p. 75 "rapid attack" shows that only with a semi automatic or an revolver you can fire 6 shots with a penalty of -4. But it contradicts the rule from page 67 about ranged attacks.
Why is it not possible with a full automatic gun ?

Well, it is possible with a full automatic weapon if it has the "Auto" designation which means it can fire in semi-automatic mode as well. Otherwise, each pull of the trigger works in full automatic only and doesn't allow for the control needed to use Rapid Attack.

As far as the contradiction goes, some maneuvers and Edges allow characters to do things that can't normally be done, but they exist as exceptions to the standard rules not necessarily contradictions.

Amphi2 wrote:

So,
can shoot more than the RoF of a gun with a "rapid attack" ?

Yes, if the weapon allows it and with the penalty to the attack rolls and Parry.

Amphi2 wrote:

do we have a penalty if we shoot more than one time per round ?

Yes. As noted, the automatic fire penalty applies to any attack with a ROF of 2+ and of course Rapid Attack has an even worse penalty.